Windmills whose rotor axes are offset sideward from the vertical axis of a mast have their rotors presented at angles out of the wind varying with wind speed. This is accomplished by a mechanism which gradually furls the rotor relative to a trailing wind-alignment vane. Such windmills have been in some use for a number of years, for water pumping and small scale generation of electricity. These windmills usually employ a relatively large number of fixed pitch blades of small diameter; however, a two-bladed fixed pitch flexible tipped rotor is shown in U.S. Pat. No. 1,818,672. The swinging motions of such windmills, caused and excited by changes in wind direction as well as by the furling and unfurling, have made it not feasible to use efficient two-bladed rotors; excessive gyroscopic and aerodynamic forces would be imposed on the larger-diameter blades, which under high, shifting winds would be likely to snap off.
In the co-pending application referred to above, it is disclosed that such gyroscopic and aerodynamic forces on rotor blade elements are nearly eliminated in rotors in which the blades passively undergo cyclic pitch change when the rotor axis is presented angularly relative to the wind. The thrust vector of the blades of such a passive cyclic pitch rotor will be directed closely along the axis of rotation even when at a substantial angle--up to say 60.degree.--to the wind direction (the aerodynamic drag of the hub itself is not, of course, so directed). The substantial coincidence of the blade thrust vector with the rotor axis despite furling, and the lack of destructive gyroscopic and aerodynamic forces on the blade elements, in combination with the other novel features of the present invention, make the passive cyclic pitch change rotor uniquely fitted for use with the offset-axis furling rotor windmill.
One potentially destructive operating condition, to which offset-axis windmills is subjected, is the uncoupling of the power extraction means during full-speed operation. This may occur, for example, during high winds, when the generator is switched off or other power-extraction means is uncoupled, either intentionally or inadvertently. Uncoupling the torque load from an offset-axis rotor may cause the rotor to so overspeed as to endanger the rotor. This may be explained as follows:
In a somewhat idealized offset-axis machine, once the rotor reaches its intended speed of rotation, its thrust is held constant. Since the machine is entirely free to yaw about the axis of the mast, the only element resisting furling to a stop is the design anti-furling moment supplied by an easily-extensible spring. The rotor-applied moment about the furling hinge cannot exceed this resisting moment; therefore, the product of the thrust force on the rotor and its moment arm about the yaw axis (ideally the fixed offset of the rotor axis) must be equal to the fixed anti-furling moment so supplied. If it is substantially constant, so will be the rotor thrust; the rotor furls to such angle out of the wind as will maintain the moment of its thrust equal to the resisting moment of the spring. However, using such a fixed pitch rotor, its thrust vector will depart from the shaft axis with increased angles of furl, making speed control difficult. Further, when the torque load on the rotating rotor is released, it can maintain its thrust force (and therefore the moment of its thrust) only by overspeeding to a much higher rate of rotation, which may endanger it.